Biometric identification and authentication systems measure physical and/or behavioral characteristics of a person and compare the characteristics to an earlier-prepared sample or “template” to determine whether the measurements were taken from the same person. This determination can be used to control access to tangible or intangible assets and resources on a per-person basis. Other systems based on a physical or informational key (e.g. an actual key, an electronic identification card, or a password) cannot restrict access to a particular person, since the person can transfer the key or password to someone else.
The measurements taken by a biometric system typically vary slightly at each measurement, so the comparison process must accommodate such slight variances. The accuracy of a biometric identification is inversely proportional to the amount of variability tolerated in a set of measurements: if more variability is permitted in measurements, then it becomes easier for an impostor to fool the system. On the other hand, as the permissible variability is reduced, legitimate users are more likely to be rejected due to normal measurement variances. Two key metrics describing the performance of a biometric identification system are the false reject ratio (“FRR” or “α,” the probability of legitimate user erroneously being rejected as an impostor), and the false accept ratio (“FAR” or “β,” the probability of an impostor erroneously being identified as a legitimate user). Administrators of biometric identification systems often tune the system's parameters to achieve a FAR and FRR that balance the security requirements of the resource protected by the system against the inconvenience to legitimate users of erroneous rejections.
One difficulty encountered in operating a biometric identification system is that of enrolling users into the system. Because of the previously-mentioned measurement variances, many systems require a new enrollee to provide a large number of measurement samples so the system can build a template that will permit accurate identifications later. Providing many samples can be tedious or inconvenient for the new user. Formal methods of evaluating samples to determine when “enough” have been collected, and of estimating identification performance that may be expected given a set of samples, may improve the usefulness and convenience of biometric identification systems.